ABSTRACT Acute respiratory distress syndrome (ARDS) is one of the most frequent causes of admission to intensive care units. The mortality rate of severe ARDS is ~40% primarily due to the lack of a clinical means for early detec- tion of ARDS, limitations of current prognostic scores for stratifying the risk of severe ARDS in patients with mild ARDS, and the lack of effective therapies. Thus, our long-term goals are to develop mechanistic-based markers for detecting and monitoring ARDS, to develop novel therapies for ARDS patients, and to improve outcomes by personalizing established or novel interventions to individuals at risk of developing severe ARDS. Single-photon emission computed tomography (SPECT) lung imaging of radiolabeled biomarkers shows unique promise as a tool for early detection and monitoring of ARDS progression. SPECT imaging with bio- markers of specific targets can be used to probe: i) intracellular glutathione content using hexamethylpropylene amine oxime (99mTc-HMPAO), and ii) endothelial cell death with 99mTc-duramycin, both targets that are altered early in the pathogenesis of ARDS. Physiologically-based pharmacokinetic modeling of SPECT imaging data provides an integrative mechanistic framework to quantify changes in the activity of the targeted cellular processes. Nuclear factor erythroid 2-related factor (Nrf2) is a redox-sensitive transcription factor that responds to oxidative stress by activating the expression of key antioxidant and cytoprotective enzymes via the Nrf2- antioxidant response element (ARE) signaling pathway. As such, Nrf2 has been shown to regulate glutathione biosynthesis, protect mitochondrial function, and inhibit apoptosis, all of which are pertinent to the pathogenesis of ARDS. Our central hypothesis is that changes in specific cellular targets that underlie early stages of ARDS can be sensitively quantified with SPECT biomarker imaging, and can be used to 1) stratify a host?s risk of developing severe ARDS, and 2) elucidate the underlying mechanisms of this risk. Thus, the specific aims are to: 1) Determine if in vivo lung uptake of SPECT biomarkers under mild ARDS conditions stratifies the risk of progression from mild to severe ARDS. 2) Evaluate the role of the Nrf2-ARE signaling pathway in hyperoxia tolerance that occurs with preconditioning. Under Aim1, SPECT biomarker imaging and PBPK modeling will be used to identify a profile of changes in activities of cellular targets that are differentially altered in rats with increased or decreased susceptibility to severe ARDS progression. Under Aim 2, we will evaluate the role of the Nrf2-ARE signaling pathway in the inducible tolerance to lethal hyperoxia that rats acquire by preconditioning. Outcomes are expected to provide i) mechanistically-based prognostic information that can be readily translated to the clinic to stratify the risk of severe ARDS development in hosts with mild ARDS, and ii) a quantitative and mechanistic understanding of the role of a key signaling pathway in ARDS progression and regression. This sets the stage for an important positive impact on ARDS patient outcomes and healthcare costs by identifying and intervening early in patients at risk for developing severe ARDS.